The endoplasmic reticulum (ER) is a membrane bound organelle that stretches throughout the cytosol while maintaining a single lumen and is composed of multiple domains including the nuclear envelope, peripheral ER tubules and sheets. The shape of the ER plays critical roles in sustaining ER function, like protein synthesis, while also providing a cellular scaffold through which other organelles interact and are supported. Additionally, the ER is responsible for trafficking up to one third of all proteins thatit synthesizes out to various cellular locations as a critical mechanism to maintain cellular health and homeostasis. This proposal aims to determine whether the distinct structural domains of the ER also play a role in regulating the trafficking of proteins through a bulk flow mechanism. This method of protein sorting acts in a non-selective method and is instead dependent on the structural features of the organelle, specifically, the ratio of membrane area to luminal volume. Protein transport through bulk flow has previously been demonstrated in other organelles such as the endosome and golgi and it is likely that the geometric constraints of the various ER domains also function to facilitate bulk flow transport of proteins. This proposal therefore aims t understand the mechanism by which ER proteins are trafficked throughout the ER and the impact that the shape of the peripheral ER has on protein trafficking and export. By altering the shape of the ER, Aim 1 of this proposal will determine 1) how the distribution of membrane and luminal proteins are altered following changes to the geometric profile of the ER and 2) how the trafficking and movement of membrane and luminal proteins are directly affected by the morphology of the ER. Results from this aim will provide mechanistic insight in the mechanisms regulating protein distribution throughout the ER and whether protein trafficking is regulated through a bulk flow dependent manner that is dependent on the geometric profile of the peripheral ER. Aim 2 of this proposal will determine how the shape of the ER regulates protein export by 1) determining the role of ER shape on the location of COPII labeled exit sites and 2) track accumulation of secretory cargo proteins at exit sites in proximity to peripheral ER sheets and tubules. Results from this aim will provide insight into how the shape of the ER can directly regulate the targeted export of proteins from the ER. At the completion of this proposal, the mechanism by which the structural profile of the peripheral ER regulates protein trafficking and export will be established.